The present invention relates to an electric switching device such as a circuit breaker, magnetic latch, shunt trip device, undervoltage relay, or overload protection circuit for an electrical load such as a motor, appliance, or electrical network. In particular, the present invention relates to a control mechanism for a circuit breaker which opens a set of contacts in response to a detection of a trip condition such as an overload, fault, or other error condition.
Circuit breakers often include a contact arm operating mechanism mechanically coupled with at least one contact arm and associated contact or a cross-bar assembly connected to the contact arms of a multi-phase circuit breaker. A trip apparatus (e.g., overload solenoid) often includes a moveable core (e.g., a plunger, a pivoting actuator arm, overload relay, or bimetal trip arrangement.) Generally, when a circuit breaker or other switch is in an overload, fault, error or other trip condition, the set of contacts is opened or the switch is otherwise open circuited when the trip apparatus activates the contact arm operating mechanism.
The trip apparatus generally includes a mechanical or electromagnetic plunger control. When the trip apparatus and control cooperate to move the plunger or actuator arm from a first position to a second position, the plunger activates the contact arm operating mechanism which opens the contacts. A mechanical plunger control may utilize a bimetal element to trigger, induce or provide the mechanical motion of the plunger. In an electromagnetic plunger control, the mechanical motion is often provided by a solenoid including a coil. When the coil is energized and maintained energized, the plunger activates the contact arm operating mechanism to open the contacts of the circuit breaker.
Conventional circuit breakers often utilize a bimetal trip arrangement to open the circuit breaker in response to a trip condition. The bimetal element is normally coupled in series with the load and the circuit breaker contacts. The bimetal element is heated by current applied to the load coupled to the circuit breaker. Accordingly, when the current applied to the load exceeds a certain threshold which indicates a trip condition, the bimetal element deforms and activates the contact arm operating mechanism, thereby directly disconnecting power to the load. Alternatively, the bimetal element may be utilized with a solenoid and disconnect current to the coil in response to the trip condition, thereby causing the circuit breaker to disconnect power to the load.
Another known type of trip apparatus includes a normally closed overload relay coupled in series with the circuit breaker. The overload relay is generally controlled by a microprocessor-based controller or other control circuit which monitors the current flowing through the circuit breaker and energizes the coil in the overload relay in response to the trip condition. Alternatively, the microprocessor-based controller may be utilized to control a magnetic latch or an electromagnetic plunger control system. The microprocessor-based controller can be configured to sense a variety of trip conditions. Based upon samples of the values of the current being applied to the load which is controlled by the switch, the microprocessor de-energizes the coil in response to the trip condition. Other microprocessor-based systems may also include temperature sensors mounted near the load. The microprocessor compares the sensed temperatures with predetermined limits and causes the switch to open de-energizing the coil when predetermined temperature limits are exceeded.
The above described systems, while providing satisfactory overload protection, are subject to a number of problems. One problem with the bimetallic based overload systems is the inability to accurately and effectively tailor the properties of the bimetallic actuator to the specifications of the load such as a motor or to the characteristics of a variety of trip conditions. Also, bimetal based overload systems waste energy because they often require relatively large amounts of current to deform. Another problem with electromagnetic based overload systems is the expense and manufacturing costs associated with the coils, magnets and overload relays. Additionally, the performance of bimetal elements as well as solenoids associated with electromagnetic plunger control systems often degrades over time. It is feasible to solve the problems with bimetallic and microprocessor based overload relays; however, these solutions may be relatively expensive and unworkable for high volume products.
Accordingly, it would be advantageous to provide a mechanical plunger control system which does not utilize a bimetal element or a solenoid.